The crushing of oil bearing vegetable material such as soybeans, rapeseeds or sunflower seeds is an energy intensive process as it involves several steps requiring mechanical and thermal energy. This energy is partially mechanical, e.g., breaking, grinding, rolling pressing and pelletizing, and partially thermal to degrade cell walls, reduce oil viscosity and adjust moisture content of any starting, intermediate or final product of the process. Before oil extraction proper, the oil bearing vegetable material must be prepared: energy must be used to rupture or weaken the walls of the oil-containing cells. For some oil bearing vegetable material, for example soybeans, a dehulling is also recommended. In the oil and fat industry, the treatments imposed to the oil bearing vegetable material before the oil extraction per se is called preparation.
The preparation preceding the oil extraction will be described in more details for soybeans which is by far the world major oil seed with a worldwide production in the range of 250 millions metric tons per year (2010, Soy Stats®, The American Soybean Association). A particularity of soybeans is that its oil and protein fractions are both of high value. Therefore, the preparation and oil extraction processes must be designed to preserve simultaneously the qualities of the extracted oil and of the remaining meal.
Soybean oil is produced predominantly by hexane solvent extraction. However, efficient hexane extraction requires a meticulous preparation of the soybeans in such a way that under normal circumstances, the solvent extraction plant can remove the oil in an efficient and economical way. One step of the preparation is the dehulling consisting in the removal of the fibrous hull surrounding the soybeans. The hull is rich in fibre and poor in oil and protein. Accordingly, the dehulling has two advantages: it reduces the amount of material that will have to be processed downstream, and it increases the protein content of the remaining meal left after the oil extraction.
Traditionally, the hull was removed by a process called now ‘cold dehulling’, but this process involves several thermal conditionings and long tempering to be worked out correctly. More recently, the ‘hot dehulling’ has been developed. In the hot dehulling, the soybeans are heated only once and tempering is eliminated. Consequently, hot dehulling is less energy demanding. This invention is concerned specifically with the hot dehulling.
Accordingly, the soybean crushing process and equipment that will be described below involves a preparation including hot dehulling, its major other steps being: cleaning, pre-heating and drying, fast surface heating, (hot dehulling), flaking and finally hexane solvent extraction.
During the cleaning step, oversized and foreign seed and/or material are removed from the soybeans. During the pre-heating and drying step, the soybeans are heated to typically 70-75° C. and the moisture reduction is of 2.5 to 3.0%. This step is often referred to “conditioning” in the industry. Typically, the moisture content of soybeans entering the preparation process is about 13% (in weight) to fall to 10.5 to 10.0% (in weight) when leaving the conditioning step. In the industry, the conditioning is realised predominantly in continuous flow deep bed dryer (“Bean heater”) where multiple steam-heated oval horizontal tubes are in contact with the soybeans. During such conditioning, a portion of the soybean's moisture migrates to the surface of the bean, making the soybean “sweat”. This surface moisture is then removed by hot air thus reducing the moisture content of soybeans. However, some moisture will also migrate and remain between the kernel and the hull. In the next step, the fast surface drying, the conditioned soybeans are subjected to blasts of dry hot air (150° C.) in a fluid bed heater resulting in a quick heating of the periphery of the soybean and in a sudden evaporation of the moisture accumulated between the hull and the kernel. During this sudden evaporation, steam pressure between the kernel and hull will make the hull fragile and also reduce its adhesion to the kernel which will facilitate the removal of the hull. Simultaneously, the fluid bed heater will furthermore remove typically 0.2 to 0.5% of moisture and increase the temperature of the soybeans to 80-85° C. In the hot-dehulling step, the soybeans weaken and non-adhering hulls are mechanically cracked typically in two to three pieces per soybean. The loose hulls are separated by aspiration. The resulting dehulled soybeans are mechanically flaked to yield flakes of about 0.3 mm of thickness. Said flakes are extracted in a hexane extractor to yield a full miscella containing about 25% of oil and solvent laden meal. Hexane is evaporated from the full miscella to yield oil and the solvent laden meal is desolventized, toasted, dryed and finally cooled to yield a meal having a protein content of typically 48% (in weight).
The total steam consumption for the preparation including the hot dehulling is about 100 kg/MT. This compares with a total steam consumption of about 150 kg/MT of steam for the preparation including the cold dehulling where two distinct heating steps and long tempering are needed.
However, even if reduced, the energy consumption for the preparation including the hot dehulling remains significant and in these days where energy cost is a major factor, should be reduced further. Consequently, energy recovery mechanism already in use in the preparation of oil bearing vegetable material should be investigated and improved or adapted for the preparation of soybeans. No process aiming at reducing further the energy demand during the preparation of soybeans has been described so far. However, an energy recovery solution is described but specifically for the preparation of rapeseed which is another major oil seed.
This energy recovery process is described in EU-project LIFE04 env/d/000051. In this energy recovery process, the thermal energy contained in the hot vapours leaving the rapeseed flake cooking step is recycled to preheat the rapeseed in the seed preheating step. This recovery process involves the scrubbing of the exhaust hot vapours leaving the meal drying to generate hot water which is then used to preheat the rapeseeds entering the process through the use of a conditioner made of vertical heat exchanger plates. The rapeseeds are flowing between the plates by gravity and are preheated by conductivity. This process allows indeed recovering thermal energy contained in the hot vapours leaving the flake cooking step; however, the scrubbing of those hot vapours generates warm water contaminated by fatty material and fines. Therefore, the cleaning and the maintenance of the equipment in contact with the warm water contaminated by fatty material and fines are difficult. Alternatively, the hot vapour stream(s) could be cleaned by filter media or cyclonic separation, but those methods are inefficient since they lead to a rapid clogging of the cleaning means due to the sticky nature of the contaminants created by the protein contained in the fines. In the previous example, some energy is recovered from the hot vapours leaving the rapeseed flake cooking step, but hot vapour leaving other step(s) of the process (such as the meal drying step for instance) would be also contaminated by fines and/or fatty materials due to the very nature of the processed vegetable material.
It is therefore the aim of the present invention to describe an oil bearing material crushing process and equipment to recover thermal energy contained in any hot vapour stream(s) contaminated by fatty material and/or fines and/or odoriferous components, such process and equipment incurring minimum fouling of heat transfer mechanisms to sustain continuous high efficiency with minimum cleaning or maintenance.